Connecting pieces are used for, for example, connecting a fitting, such as a tap, to a pipe or connecting a pipe to another pipe. If, for instance, a plastic pipe is to be connected to a metal pipe or a metal fitting, it is typical to use a connecting piece with a plastic body to the first end of which a plastic pipe is connected. The second end of the plastic body, in turn, is equipped with a metal insert provided with a thread, to which a metal pipe or a metal fitting is connected. Such solutions are disclosed for example in DE 8 908 805, EP 0 744 573, JP 10 103 569, JP 09 250 675, EP 0 556 767, U.S. Pat. No. 4,790,573, DE 19 510 714, EP 0 493 316 and U.S. Pat. No. 5,437,481.
Also EP 0 565 957, CH 675902, EP 0 531 709 and DE 8 406 562 disclose connecting pieces in which there is a metal insert arranged inside a plastic body. The outer surface of the metal insert is formed hexagonal, whereby this hexagonal outer surface can be gripped with a tool upon making the threaded connection.
EP publication 0 870 970 discloses an adapter with which a pipe of a water system, for example, can be connected to a system element. The adapter comprises a body having a seat for a pipe and a metal insert provided with a thread for connecting it to the system element. The outer surface of the adapter is provided with longitudinal ribs and a robust external thread to which a ring nut can be screwed to tighten the connection between the adapter and the pipe.
DE publication 3 306 192 discloses a connecting piece by means of which a plastic pipe is connected to a metal fitting. The interior of the plastic body of the connecting piece is equipped with a metal insert provided with a thread, the fitting being attached in the insert. The outer surface of the plastic body is provided with grooves, which makes it easier to keep the connecting piece immovable when the fitting is screwed to the connecting piece.
DE publication 4 430 435 discloses an elbow-like connecting piece for connecting a pipe to a metal fitting. There is a bend part in the connecting piece, the first end of the bend part comprising a socket for attaching the pipe and the second end comprising a socket having an insert provided with a thread. The outer surface of the second end comprises longitudinal ribs.
DE publication 2 626 302 mentions a connecting piece made of synthetic resin, having an external thread at the first end for connecting the connecting piece to a metal pipe, for instance. The second end of the connecting piece, in turn, comprises a tightener part for connecting the connecting piece to a pipe made of synthetic resin. On the outer surface of the connecting piece there is a nut, whereby the connecting piece can be easily gripped with a tool. Disadvantages of such a solution have been reported to be that the external thread part gets easily broken and damaged, whereby the external thread connection begins to leak, and in addition, the nut part wears and gets slanted easily. As a solution to these problems, the publication suggests that the external thread part and the nut part of the connecting piece be made of metal.